Diplomats also reported that several Security Council members - including Russia, China, Indonesia and Qatar - said they had no way of independently ascertaining where the incident took place and were therefore wary of condemning it.

Tony Blair, the British prime minister, said he was disgusted by Iran's treatment of the prisoners.

"Obviously I felt the same way most people do, which is a sense of disgust that people would be used in that way," he told ITV news on Thursday.

"What I'm afraid we can't do is end up in negotiation over hostages. What we can't do is say there's some kind of quid pro quo or tit-for-tat that goes on.

"This is not a situation that can be resolved by anything other than the unconditional release of all our people."

Stilted English

Iran has shown the prisoners on television, and on Thursday distributed a second letter purportedly from the only female captive, Faye Turney, confessing to entering Iranian waters.

Both letters were in stilted English, with unusual phrases that linguistic experts said appeared to have been translated from Farsi into English.

"Unfortunately during the course of our mission we entered into Iranian waters. Even through our wrongdoing, they have still treated us well and humanely, which I am and always will be eternally grateful," Thursday's letter said.

It called for British forces to withdraw from Iraq.

Beckett's reaction

Margaret Beckett, the British foreign secretary, responded in a statement: "We have not seen this letter but we have grave concerns about the circumstances in which it was prepared and issued.

The captured British servicemen were fromthe warship HMS Cornwall [AFP]

"This blatant attempt to use leading seaman Turney for propaganda purposes is outrageous and cruel."

Iran had said on Wednesday that it would free Turney soon. But on Thursday Alireza Afshar, the Iranian military commander, said her release had been "suspended".

"The wrong behaviour of those who live in London caused the suspension," he said, adding that Britain must apologise for entering Iran's waters and promise it would not happen again.

Turkish mediation

Meanwhile, Iranian state television reported that Recep Tayyip Erdogan, the Turkish prime minister, had urged Mahmoud Ahmadinejad, the Iranian president, to allow Turkey access to the seized troops and to free Turney.

The channel said Ahmadinejad would consider the Turkish request.

The Iranian president also reportedly accused Britain of using propaganda in the case rather than trying to solve it through diplomatic channels.

Turkey maintains good relations with Iran and the West.

Separately, the US said on Thursday that two aircraft carrier groups were in the Gulf not to provoke Tehran but to reassure friendly governments in the area.

Nicholas Burns, the undersecretary of state, said in testimony to the Senate Foreign Relations Committee: "We are not there to provoke any military conflict."